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Cacat Pada Kristal Logam 1

Dislokasi Kristal

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  • Cacat Pada Kristal Logam

    1

  • 2

    Crystals are like people, it is the defects in them

    which tend to make them interesting! - Colin Humphreys.

    Cacat kristal (Defects) di dalam logam:

    0D, Cacat titik (Point defect)

    vacancies

    interstitials

    impurities, weight and atomic composition

    1D, Dislokasi/cacat garis (Linear Defect)

    2D, Cacat Bidang (Planar Defect)

    3D, Cacat Volume (Volume Defect)

  • 3

    Kristal yang sebenarnya (Real crystals) tidaklah

    pernah sempurna karena didalam strukturnya selalu

    terdapat cacat (defect)

    Schematic drawing of a poly-crystal with many defects

    by Helmut Fll, University of Kiel, Germany.

    Cacat Kristal (Defect)

  • 4

    Why Study Defects?

    Defects memiliki pengaruh yang sangat besar dalam menentukan sifat suatu material

    Sebagai contohnya: sifat mekanik dari logam murni sangat berbeda jika dibandingkan dalam

    bentuk paduan (alloy). Perunggu (70% Cu- 30%

    seng) dengan Cu murni yang bersifat lunak

    Contoh lainnya: komponen IC (semikonduktor) didalam perangkat elektronik sangat dipengaruhi

    oleh konsentrasi impurity di dalamnya.

  • 5

    Jenis Defect

    Jenis-Jenis Defect Berdasarkan

    Dimensinya:

    0D, Point defects: Cacat titik yang paling sederhana adalah

    kekosongan disebabkan adanya atom yang hilang

    dalam kristal (ex: Vacancy, Interstitial, Impurity)

    1D, Linear defects: Adanya sisipan satu bidang atom tambahan

    didalam struktur kristal akibat tekanan dan

    tegangan (ex: edge, line, screw dislocation)

    2D, Planar defects: Cacat yang ditandai adanya pemisahan batas 2D

    yang memiliki kristal berbeda (ex: grain

    boundaries, external surfaces)

    3D, Volume defects: extended defects (ex: pori, retak/crack, inklusi)

  • 6

    Point defects: vacancies &

    interstitials

    Self-interstitials

    Vacancy

    Vacancy - kisi yang kosong akibat kehilangan

    atom

    Interstitial adanya atom yang menempati

    celah kosong diantara atom. Defect jenis ini

    dapat berupa atom yang sama (self interstitial)

    atau interstisial dari atom pengotor

  • 7

    Jumlah ruang kosong pada kondisi

    equilibrium (Vacancy) akibat vibrasi

    termal:

    Bagaimana Jumlah Vacancy di

    hitung?

    Ns = jumlah regular lattice

    kB = konstanta Boltzmann

    Qv = energi yang diperlukan untuk membentuk

    ruang kosong pada kisi kristal ideal

    T = temperature dalam Kelvin.

    At room temperature in Cu: one vacancy per 1015

    atoms.

    Just below the melting point: one vacancy for every

    10,000 atoms.

    Tk

    QexpNN

    B

    vsv

  • 8

    Estimate number of vacancies in Cu at room T

    kB = 1.38 10-23 J/atom-K = 8.62 10-5 eV/atom-K

    T = 27o C + 273 = 300 K.

    kBT = 300 K 8.62 10-5 eV/K = 0.026 eV

    Qv = 0.9 eV/atom

    Ns = NA/Acu

    NA = 6.023 1023 atoms/mol

    = 8.4 g/cm3

    Acu = 63.5 g/mol

    Tk

    QexpNN

    B

    vsv

    3

    223

    23

    s cmatoms108

    molg

    5.63

    cmg

    4.8mol

    atoms10023.6

    N

    atomeV026.0atomeV9.0

    expcm

    atoms108N

    3

    22

    v

    37 cmvacancies104.7

  • 9

    Distorsi besar dalam kisi kristal di sekitarnya

    Energy pada self-interstitial formation

    ~ 3 x lebih besar daripada vacancies (Qi ~ 3Qv)

    Konsentrasi atom pada self-interstitials sangat

    kecil (< 1/ cm3 at 300K)

    Self-interstitials

    1

    2

    3

    4

    5

    Point defects: self-interstitials, impurities

    (1) vacancies

    (2) self-interstitial

    (3)interstitial impurity

    (4,5)substitutional

    impurities

    panah arah stress lokal yang diakibatkan

    oleh defects

  • 10

    Impurity

    Impurity atom pengotor yang ada di dalam material

    Semua logam sebenarnya tidaklah murni. Very pure metals 99.9999%

    - one pengotor (impurity) per 106 atoms

    Adanya pengotor mungkin disengaja atau tidak

    Carbon in small amounts in iron makes steel. It is stronger.

    Boron in silicon change its electrical properties.

    Alloys paduan dua logam atau lebih

    Sterling silver dibuat dari 92.5% silver 7.5% copper alloy. Sifatnya lebih kuat daripada perak

  • 11

    Bagaimana Ada Impurity dalam logam?

    Solid solutions

    Host (Solvent) melarutkan komponen dalam jumlah kecil (Solute).

    Kemampuan untuk melarutkan disebut Solubility.

    Solvent: komponen terbesar dalam campuran

    Solute: komponen dalam jumlah kecil di campuran

    Solid Solution: homogeneous

    maintain crystal structure

    randomly dispersed impurities

    (substitutional or interstitial)

    Second Phase

    solute atoms added: new compounds or structures form or solute forms local precipitates

    Sifat larutan padat dipengaruhi oleh impurities, concentration, temperature and pressure

  • 12

    Larutan Padat Subsitusi (Substitutional

    Solid Solutions)

    Factors yang mempengaruhi kelarutan

    atom:

    (Solubility limit maximum dapat larut)

    Atomic size: need to fit solute and solvent atomic radii should be within ~ 15%

    Crystal structure: solute and solvent the same

    Electronegativities: should be comparable (otherwise new inter-metallic phases

    favored)

    Valency: If solute has higher valency than solvent, generally more goes into solution

    Ni

    Cu

  • 13

    Larutan Padat Interstisi (Interstitial Solid

    Solutions)

    Factors for high solubility:

    FCC, BCC, HCP: void space between host (matrix) atoms relatively small

    atomic radius of solute should be

    significantly less than solvent

    Max. concentration 10%, (2% for C-Fe)

    Carbon

    interstitial

    atom in BCC

    iron

    Interstitial solid solution of C in BCC Fe ( phase).

    C small enough to fit (some strain in BCC lattice).

  • 14

    Composition / Concentration

    atom percent (at %): useful in understanding material at atomic level

    Number of moles (atoms) of one element relative to

    total number of moles (atoms) in alloy.

    2 component: concentration of element 1 in at. %:

    Weight Percent (wt %)

    Weight of one element relative to total alloy weight

    2 components: concentration of element 1 in wt. %

    100mm

    mC

    21

    11

    100nn

    nC

    21

    1

    mm

    m'

    1

    nm1= number density = m1/A1 (m1 = weight in

    grams of 1, A1 is atomic weight of element 1)

  • 15

    Dislocations = Linear Defects

    Interatomic bonds significantly distorted in

    immediate vicinity of dislocation line

    (Creates small elastic deformations of lattice at

    large distances.)

    Dislocations affect mechanical properties

    Discovery in 1934 by Taylor, Orowan and Polyani

    marked beginning of our understanding of

    mechanical properties of materials

  • 16

    Interfacial Defects External Surfaces

    Surface atoms unsatisfied bonds

    higher energies than bulk atoms

    Surface energy, (J/m2)

    Surface areas try to minimize (e.g. liquid drop)

    Solid surfaces can reconstruct to satisfy atomic

    bonds at surfaces.

    Grain Boundaries

    Polycrystalline: many small crystals or grains.

    Grains have different crystallographic orientation.

    Mismatches where grains meet.

    1. Surfaces and interfaces are reactive

    2. Impurities tend to segregate there.

    3. Extra energy associated with interfaces

    larger grains tend to grow by diffusion of atoms at expense of smaller grains, minimizing energy.

  • 17

    High and Low Angle Grain Boundaries

    Misalignments of atomic planes between grains Distinguish low and high angle grain boundaries

  • 18

    Bulk or Volume Defects

    Pores: affect optical, thermal, mechanical properties

    Cracks: affect mechanical properties Foreign inclusions: affect electrical,

    mechanical, optical properties

    Cluster of microcracks in a

    melanin granule irradiated

    by a short laser pulse.

    Computer simulation by L.

    V. Zhigilei and B. J.

    Garrison.